How a Tiny, Low-Power MCU Meets the Needs of an Optical Module Design

How a Tiny, Low-Power MCU Meets the Needs of an Optical Module Design



The advent of 5G heralds the era of the technology Internet of Things. Although the end user is connected to the network wirelessly, the core of the network needs stable and reliable wired connection so fiber optic is a typical way of interconnection. This article describes Maxim's microcontroller to design an optical module which is an essential part of fiber optic communication.

5G is a hot topic nowadays, and the arrival of 5G foreshadows a new era of the "Internet of Things." The 5G network that makes this possible is expansive, featuring many terminals which connect to the network wirelessly. However, area networks need to connect to the internet by wired means, and optical fiber communication is a typical interconnection method where optical modules are a very important and essential part. This article discusses how a tiny, low-power microcontroller unit (MCU) plays an important role in such applications.

Maxim Integrated's MAX32660 is ideal for today's optical module designs based on features and functions such as:

  • High Performance:
    • Built-in 96MHz Clock Source
    • Arm® Cortex®-M4F Core
    • Internal 256kB Flash
    • 96kB SRAM, with Retention Options at Low-Power Modes
    • 16kB Instruction Cache
    • Memory Protection Unit (MPU)
    • 0.9V to 1.1V Core Voltage
    • 1.7V to 3.6V GPIO Operating Range
    • Integrated LDO Operates from Single Supply
    • Industrial Operation Temperature: -40°C to +105°C
    • Optional I2C, SPI & UART Bootloader
  • Low Power:
    • 85µA/MHz Active Executing from Flash
    • 2µA Full Memory Retention Power in Backup Mode at VDD = 1.8V
    • RTC Power Consumption 570nA at VDD = 1.8V
    • Internal 80kHz Ring Oscillator
  • Optimized Peripherals:
    • Up to 14 GPIOs
    • Up to Two SPI Connections
    • Up to Two UARTs
    • Up to Two I2C Connections, 3.4Mbps High Speed
    • I2C Interface
    • Four-Channel Standard DMA Controller
    • Three 32-Bit Timers
    • Watchdog Timer
    • CMOS-Level 32.768kHz RTC Output

    The following figure is the internal block diagram of this MCU:

    Figure 1: MCU Internal Block Diagram.

    Figure 1: MCU Internal Block Diagram.

    As shown from the block diagram and the previous description, the main advantages of the MAX32660 are its high performance, low-power consumption, and small package, which makes this MCU well suited for an optical module design. Optical modules generally need two I2C interfaces: one serving as the I2C secondary interface connected to the outside and the other as the I2C primary interface that communicates with the analog front-end (AFE).

    While the MAX32660 has the smallest package and the fewest GPIOs in Maxim Integrated's MCU family, this does not mean that it is weak in any way. Like other MCUs with abundant peripherals, it has a high clock speed of 96MHz and a Cortex-M4F core; often times customers only find it suitable for applications that allow them to run propriety algorithms with few peripherals. However, in optical modules the microcontroller must have common peripherals like I2C in addition to fast processing speeds at the core. Although the MAX32660 has fewer I/O ports, it is optimized with all of the common interfaces.

    Optical modules consist of optoelectronic devices, functional circuits, and optical interfaces. Optoelectronic devices have transmitting and receiving modes. In short, the function of optical modules is photoelectric conversion; the transmitter converts the electrical signal into an optical signal, and then the receiver converts the optical signal into an electrical signal after transmitting it through an optical fiber.

    During transmission, the electrical signal with a specific bit rate is input and processed by the internal driving chip to drive the semiconductor laser (LD) or light emitting diode (LED) to emit the modulated optical signal with the corresponding bit rate. Moreover, the internal optical power automatic control circuit is provided to keep the output optical signal power stable.

    For the receiving mode, the optical signal with a certain bit rate is input into the module and converted to an electrical signal by the light detection diode, which is amplified by the preamplifier to output the electrical signal with the corresponding bit rate.

    Optical modules can be classified by functions, parameters, and packages, which are not described in detail in this article. Those who are interested can find additional resources online.

    For the low-end optical module, the signal is directly and photoelectrically converted and the bit rate of the output electrical signal is identical to that of the optical signal. There are many high-speed optical modules which convert multiple electrical signals into one optical signal. The DSP, a device that consumes a high amount of power, is used to process data for bridging.

    The following is the internal block diagram of a typical optical module:

    Figure 2: Typical Optical Module Internal Block Diagram.

    Figure 2: Typical Optical Module Internal Block Diagram.

    As shown in the previous figure, the MCU manages many peripheral devices through I2C and also acts as an I2C secondary to communicate with the host. The MAX32660 supports up to 3.4Mbps I2C speed, which can meet the communication speed requirements of a large majority of hosts.

    The MAX32660 works well with Maxim Integrated's companion AFE chips, both of which form an ideal combination. This MCU operates through I2C, SPI, or UART ports to control the AFE's ADCs, TEC drivers, etc.

    TEC stands for thermal electronic cooler and can be regarded as a chip-level coolant, which plays an important role in the optical module. In the transmission part of the optical module, the laser emits light signals and needs to be cooled during operation to ensure stability. The input current and voltage signals must be accurate and monitored by an ADC while the laser is operating.

    The MCU is the core of the entire system; since it coordinates with other devices, it needs to have relatively high processing power and certain peripheral interfaces. The MCU needs to set and monitor the normal operation of each functional circuit. For example, the MCU supports real-time communication with the DSP and AFE chip, monitors the temperature, voltage, and current of the DSP and laser, and responds to host communication in real time.

    Maxim Integrated's MCUs have a relatively high frequency, and the Cortex-M4F core can ensure that these transactions can be processed in real time. At the same time, optical module customers generally need an MCU with an online upgrade function so that the host can upgrade the MCU online through the I2C interface. Maxim Integrated has developed a bootloader which can communicate through I2C, UART, or SPI while providing customers with the bootloader source code for easy implementation. This gives customers the option to choose between using Maxim Integrated's bootloader or designing their own.

    To meet the demands of optical modules for continuous upgrades and function enhancement, Maxim Integrated built on the MAX32660 and added the MAX32670, which integrates more memory (flash/SRAM) with enabled error correcting code (ECC) for applications where operational reliability is crucial and where more memory-intensive algorithms are required. Also, the MAX32670 integrates a full suite of security features and a secure bootloader.

    In conclusion, it's important to note that Maxim Integrated has many years of experience in the field of optical modules and a deep understanding of the industry. Maxim Integrated offers a wide variety of optical module products such as MCUs, optical AFEs, ADCs, DACs, DC-DC, TEC drive, and more—all resulting in an easier and faster integration of products.

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